Because they got in touch with American Blacks and picked up bad habits. It's part of that Urban/Gangster culture. Not all Blacks follow it, but that culture is closely tied to the Black community, slavery, Rap music, sagging the pants like in prison, etc. White people here who follow it will often claim they have "Black blood in them", if anyone questions their behaviour or a Black person accuses them of cultural appropriation. LOL.
Black people who don't follow that culture tend to hate being associated with it because of their race, though. So it's definitely not all Black people that act this way, although I have heard Black people condemn other Blacks for "acting White" if they try to go against this culture.
I'm not really a fan of that subculture, myself. I prefer to be around people who speak English clearly, wear undamaged clothing in a correct manner, and listen to Classical or Metal music. Oh, and I'm apparently racist for not liking Rap music. Yes, someone actually said that to me. I hope they were kidding, but I'm really not sure.